In the field of HF communications, HF links offer notably a beyond-the-horizon capability, better known by the acronym BLOS (Beyond Line of Sight), which enables the implementation of long-distance or very long distance communications without the need to use a satellite or relay stations. One of the problems posed in these communications is knowing how to speed up the link establishment in synchronous mode or in asynchronous mode in order to offer a shorter link establishment time and therefore a greater ease of use for the user, and/or a faster service. A plurality of techniques are described in the prior art that speed up connections for these two modes.
For example, in the case of an asynchronous link, the aforementioned patent application of the Applicant recommends the implementation of parallel monitoring on the receiver side.
Another improvement consists in adopting a synchronous link establishment, i.e. based on a common time reference, or coordinated universal time (UTC) or through the sharing of a time reference provided by a master clock on the network. The synchronous method is considered to be the most effective to date. This solution has improved the link establishment time, in the context of generally low to medium network load. Since a standard narrowband terminal automatically adheres to the synchronous monitoring plan by definition, it does not seem possible to propose significant improvements in this technique, except:                1) by reducing the number of frequencies of the plan, since the synchronous solution gives an average connection time proportional overall to the number of frequencies of the plan concerned,        2) by carrying out multiple listening in the same time increment. Patent EP 2 418 892 implements a link establishment with a bandwidth expanded from 6 to 24 KHz based on a frequency F0.        
The techniques currently offered by the prior art have different disadvantages.
In the case of asynchronous solutions, there is a convergence time problem when substantial frequency plans are involved, since the call duration is proportional to the number of frequencies of the plan. Lower performance may also occur under low to medium load compared with a synchronous method.
In the case of synchronous 3G solutions, the known techniques do not result in an identified potential gain, except by reducing the number of frequencies. In this case, however, there are fewer possible frequencies, with a risk of not being able to establish the link or “no link establishment” if the frequencies are non-passing, are already in use or are affected by interference.
The multiple-listening solution in the same time increment yields no gain in synchronous mode, since the transmitter transmits on one frequency only. The resulting disadvantages are a risk of loss of sensitivity of the broadband receiver in the presence of interfering transmitters or for the call.
In the description below, the term “legacy” refers to a historic system or standard based on single-band transmissions (or through combination of a plurality of adjacent bands), for example, or, more generally, narrowband transmissions (i.e. several kHz to several tens of kHz). The calling terminal corresponds to the communication terminal or device which initiates the call request. The called terminal is the terminal that receives this call request.